1. Field
The present disclosure relates to an electrophotographic image forming apparatus and more particularly, to an electrophotographic image forming apparatus using a contact charging device.
2. Description of the Related Art
A charging device having the shape of a roller using a rubber layer around a shaft is used as a contact charging device. The roller is in pressure contact with a photoreceptor drum by own weight or a load to form a microscopic gap and a surface of the photoreceptor drum is uniformly charged by electrical discharge from a portion thereof. The drum, for example, is formed of an organic photo-conductor (OPC).
A charging method may be classified as an alternating current (AC) charging method, in which an alternating current is in superposition of a direct current (DC), and a DC charging method using only a direct current. The AC method has excellent stability, but has high discharge stress, and the AC method is disadvantageous in view of lifetime accompanying the generation of discharge products or deterioration of the surface of a photoreceptor drum. The DC method has low stability, but is excellent in view of total costs including the lifetime and environmentally friendly characteristics (so-called “eco characteristics”). Recently, the DC charging method typically limited to a low-printing rate region has been deployed in a high-printing rate region.
In general, a charging roller is polished by sliding a whetstone in an axial direction when the shape thereof is trimmed in a fabrication process. At this time, in order to more stably maintain a nib, uniform charging is performed by using a roller having low surface roughness, in which roughness is minimized by the improvement of polishing accuracy or coating.
A surface of the charging roller is contaminated by external additives of a toner. Charge stains that are generated due to the contamination appear as vertical stripes in an image. In order to remove the vertical stripes, cleaning is performed by allowing a sponge roller to be rotated in contact with the charging roller.
For example, a charging roller according to the prior art is described in Japanese Patent Publication No. 08-062949.
With respect to DC charging, since chargeability is low, many lateral stripes are frequently generated in a halftone image. These microscopic stains are denoted as microjitter. Microjitter is caused by microscopic charge defects or electrical discharge due to polishing marks on the surface of the charging roller.
In particular, with respect to a type in which a charging roller is in pressure contact with a photoreceptor drum, electrical discharge or charge defects are more facilitated due to the instability of the nib. Therefore, a measure of reducing the possibility of the generation of electrical discharge or charge defects by uniformizing the nib through smoothing the surface thereof as much as possible was typically selected. However, when the surface is smoothed, a decrease in a rotation performance of the charging roller driven by the photoreceptor or an effect of peeling electrification may not be ignored.